Aquatic farming has long recognized the importance of the water quality in raising aquatic life such as fish. Among the variables associated with water quality as it relates to the survival, growth, and management of fish are temperature and dissolved oxygen content. These two interrelated variables may, independently or in combination, result in fish kills or damage to the organism in a very short time period, therefore aquatic farming has heretofore been a labor intensive undertaking due to the obvious requirement that these conditions be closely monitored and controlled where possible.
In as much as bodies of water are generally heated from the surface, the surface waters heat faster than the lower waters with a marked decrease in density as the temperature rises such that the surface waters can become so warm and light that they do not mix with the cooler, heavier water in lower layers. This phenomenon, known as thermal stratification, yields an upper layer of warm water, the epilimnion, a lower, cooler layer known as the hypolimnion, and a layer having a rapidly changing temperature between the epilimnion and the hypolimnion. Warm water fish grow best at temperatures between 25.degree. and 32.degree. C., but are intolerant to rapid temperature changes even within this range, thus if the epilimnion becomes overheated, fish seeking relief in the hypolimnion may be subject to stress as they encounter the temperature changes.
Dissolved oxygen is probably the most critical variable. The solubility of dissolved oxygen is inversely variable with the temperature and diffusion is quite slow, therefore photosynthesis by phytoplankton is the primary source of oxygen in the system and light is often the primary factor regulating photosynthesis by phytoplankton. Light rapidly decreases in intensity as it passes through water, consequently the rate of oxygen production by phytoplankton decreases with depth and, below a certain depth, no more oxygen is produced. Since oxygen is constantly used by the aqualife and since oxygen is only produced by photosynthesis during daylight, there is a depth in stratified ponds where the water will contain no dissolved oxygen. The stratification of dissolved oxygen usually corresponds closely to thermal stratification with the epilimnion containing dissolved oxygen and the hypolimnion being depleted of dissolved oxygen.
Representative of the prior art known to me as having attempted to deal with the problems of water quality management are U.S. Pat. No. 3,698,359 to Fremont and U.S. Pat. No. 4,516,528 to Jones. Fremont teaches a large floating enclosure formed by a closed membrane within which water quality is controlled by pumping in oxygenated and treated water. Displaced water exits via gutters at the periphery of the enclosure. Jones teaches an elaborate aeration raceway system.
Of course, numerous pumps, sprayers, and paddlewheel aerators have also been used in aquatic farming, however these are often time and labor intensive.